The measurement of water in gases is critical in chemical process industries because awareness and control of the water content in feed streams, reaction mixtures and product streams often determines the efficiency of a manufacturing process, quality of the product, and overall success of the process. The need to monitor chemical process streams and products either continuously or in batch operations has led to the development of a variety of sophisticated analytical instruments for such purpose. However, many of these instruments are highly complex and expensive, lack portability or longevity, are unable to measure small amounts of water directly and reliably, or exhibit a response time which is too slow for many applications.
Electrolytic measurement of water is extensively reviewed in Smith & Mitchell, Aquametry: Part II, Electrical and Electronic Methods--A Treatise on Methods For The Determination of Water, Second Edition, John Wiley & Sons (1984), 1352 pages, especially Section III: Coulometric Hygrometry, pages 51514 -991. A well-known electrolytic cell described in the Smith & Mitchell text, in U.S. Pat. Nos. 2,816,067 and 2,830,945 to F. A. Keidel and in F. A. Keidel, "Determination of Water by Direct Amperometric Measurement", Analytical Chemistry 31, 2043-2048 (1959) utilizes electrode wiring in a tubular conduit with a phosphoric acid-derived coating over the wires to absorb water in a gas passed through the conduit. The absorbed water is electrolyzed by application of a suitable potential across the wires. Since electrical current consumption in the electrolysis of the water removed from the gas is governed by Faraday's Law, the quantity of water in the gas is directly measurable. (The foregoing publications and patents are incorporated herein by reference).
Although the Keidel cell is portable, the versions of the cell developed to date exhibit slow response times when the cell is dried down to a baseline dry state between determinations, requiring on the order of 1 to 5 hours to measure reduction of water in a dry inert gas stream from 5 to 1 ppmv or less. The "dry down" response time reflects and is a measure of the response time during continuous operation of the cell. In addition, continuous monitoring of modern process streams and reduction of downtime for correction of composition and flow rate of the streams requires cells having superior longevity and durability. While a longevity of several days may be adequate in some systems, capability of operation for weeks and even months can be a significant contributor to the efficiency and economy of a manufacturing sequence and quality of the product. Accordingly, the dominant criteria for an improved water measurement system are sensitivity to low water content, of the order of about 2.0 to 0.005 ppmv, fast response time for measurement of the low water levels, and extended system longevity, of the order of several weeks to months. Other important criteria include portability of the measuring apparatus and capability of making water determinations directly, i.e., without need for comparison with a standard.